Optical transmission systems have enjoyed increasing usage throughout telephone network and switching systems. For example, optical fiber systems have replaced copper based transmission systems in many trunk and junction networks. As optical fiber systems become more pervasive in the user area of the telecommunication system, better and less expensive usage of the capabilities of optical fiber transmission systems will be in demand.
Within the user area, a contemplated application of optical transmission systems is for connection to a broadband integrated services digital network ("BISDN") for providing interactive services including the distribution of video signals in the local loop. The optical transmission system configuration envisioned for such a connection network is a shared medium passive optical network ("PON") having a tree-and-branch topology. This topology provides a cost-effective way to connect small businesses and residential subscribers to an BISDN, and has been used in several field trials for Plain Old Telephony Services and distributive services, such as the Amsterdam-Sloten Fibre to the Home Field which is described in H. H. Grotjohann, F. Jaeger and P. E. Schaafsma, Dutch FTTH on Trial, Communications International, p. 49 (January 1992).
In a PON, the optical fiber is shared by a group of customers. Further, in a PON having a tree-and-branch topology, a single fiber emanates from a local exchange and fans out via passive optical splitters and tree couplers to a plurality of service customers.
One conventional method for transmitting information from the local exchange to individual customers utilizing such a topology is as follows. Information is transmitted in packets or cells. Each transmitted cell contains data and address information of the intended destination customer. All information cells are transmitted through the entire PON to receivers located at the ends of the network which are called network terminations. Each network termination is connected to a corresponding customer or group of customers. When a network termination receives an information cell, it compares its address with the address contained within the cell. When the addresses match, the cell is passed to the proper customer. When a match is not detected, the network termination blocks the received information and does not transmit it to the customer.
Encryption of the data contained in information cells has been used to avoid the potential problem of eavesdropping by unscrupulous customers tapping into their network terminations. If the data was not encrypted, an unscrupulous customer would have access to all the information transmitted on the PON by the local exchange. Encrypted information cells, on the other hand, are properly deciphered only by the intended destination network termination. To insure this result, the same key is used by the LT and destination NT for encryption and decryption, respectively. A different key is associated with each NT.
One conventional method of encryption is to combine an information cell with a pseudo-random stream of binary bits using modulo 2 addition. Generation of the pseudo-random sequence can be accomplished by dedicated hardware, such as feedback shift registers, or in software. Deciphering the encrypted information stream occurs by combining, using modulo 2 addition, the encrypted information cell with the identical pseudo-random sequence used for encryption.
Some prior art systems have utilized encryption on all parts of the information cell except for the routing address information. As a result, conventional methods of encryption of data cells would not prevent an unscrupulous customer from obtaining information concerning the frequency of transmissions to or from the other customers on the network.
Encryption of the address information is desirable to insure privacy of traffic behavior of customers on the network. However, typical prior art systems have not encrypted the address information because of the inherent problem of extraneous transmissions of encrypted information cells to non-destination customers. These extraneous transmissions or misdeliveries present a security risk to the data contained in the information cells, as well as a burden to the control circuitry within the non-destination NTs. Misdeliveries occur when non-destination network terminations use their respective decryption patterns on the routing address information of the received information cells, and inadvertently produce addresses that are equal to their own. As a consequence, the still encrypted cells will be transmitted to unintended customer premises networks.